Artist's impression of the ten hot Jupiter exoplanets studied by David Sing and his colleagues. From top left to to lower left these planets are WASP-12b, WASP-6b, WASP-31b, WASP-39b, HD 189733b, HAT-P-12b, WASP-17b, WASP-19b, HAT-P-1b and HD 209458b. Credit: ESA/Hubble & NASA

Astronomers have used the NASA/ESA Hubble Space Telescope and the NASA Spitzer Space Telescope to study the atmospheres of ten hot, Jupiter-sized exoplanets in detail, the largest number of such planets ever studied. The team was able to discover why some of these worlds seem to have less water than expected—a long-standing mystery. The results are published in Nature.

To date, astronomers have discovered nearly 2000 planets orbiting other stars. Some of these planets are known as hot Jupiters, hot, gaseous planets with characteristics similar to those of Jupiter. They orbit very close to their stars, making their surface hot, and the planets tricky to study in detail without being overwhelmed by bright starlight.

Due to this difficulty, Hubble has only explored a handful of hot Jupiters in the past, across a limited wavelength range. These initial studies have found several planets to hold less water than expected.

Now, an international team of astronomers has tackled the problem by making the largest ever study of hot Jupiters, exploring and comparing ten such planets in a bid to understand their atmospheres. Only three of these planetary atmospheres had previously been studied in detail; this new sample forms the largest ever spectroscopic catalogue of exoplanet atmospheres.

The team used multiple observations from both the NASA/ESA Hubble Space Telescope and NASA's Spitzer Space Telescope. Using the power of both telescopes allowed the team to study the planets, which are of various masses, sizes, and temperatures, across an unprecedented range of wavelengths.

"I'm really excited to finally 'see' this wide group of planets together, as this is the first time we've had sufficient wavelength coverage to be able to compare multiple features from one planet to another," says David Sing of the University of Exeter, UK, lead author of the new paper. "We found the planetary atmospheres to be much more diverse than we expected."

This image shows an artist’s impression of the ten hot Jupiter exoplanets studied by David Sing and his colleagues. The images are to scale with each other. HAT-P-12b, the smallest of them, is approximately the size of Jupiter, while WASP-17b, the largest planet in the sample, is almost twice the size. The planets are also depicted with a variety of different cloud properties. Credit: ESA/Hubble & NASA

All of the planets have a favourable orbit that brings them between their parent star and Earth. As the exoplanet passes in front of its host star, as seen from Earth, some of this starlight travels through the planet's outer atmosphere. "The atmosphere leaves its unique fingerprint on the starlight, which we can study when the light reaches us," explains co-author Hannah Wakeford, now at NASA Goddard Space Flight Center, USA.

These fingerprints allowed the team to extract the signatures from various elements and molecules—including water—and to distinguish between cloudy and cloud-free exoplanets, a property that could explain the missing water mystery.

The team's models revealed that, while apparently cloud-free exoplanets showed strong signs of water, the atmospheres of those hot Jupiters with faint water signals also contained clouds and haze—both of which are known to hide water from view. Mystery solved!

"The alternative to this is that planets form in an environment deprived of water—but this would require us to completely rethink our current theories of how planets are born," explained co-author Jonathan Fortney of the University of California, Santa Cruz, USA. "Our results have ruled out the dry scenario, and strongly suggest that it's simply clouds hiding the water from prying eyes."

The study of exoplanetary atmospheres is currently in its infancy, with only a handful of observations taken so far. Hubble's successor, the James Webb Space Telescope , will open a new infrared window on the study of exoplanets and their atmospheres.

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13 comments

"We found the planetary atmospheres to be much more diverse than we expected."

It appears we are in golden age for astronomy. Telescopes and simulations are wonderful tools, but ultimately they are limited. We will not be able to fully understand the galaxy without traveling through it. Eventually, interstellar travel will be a scientific imperative, not a scientific impossibility. Go boldly.

Eventually, interstellar travel will be a scientific imperative, not a scientific impossibility.

I would strap myself to a warp cannon if half the world's scientists were convinced it would blow up on the launch pad, so long as the other half thought it might accidentally work in some parallel universe where wishes are horses. I don't care if the odds of success being above zero are dependent on infinity existing, just install a seat belt so people wont complain about safety.

Of course this ignores the fact that NASA has five (5) spacecraft bound for interstellar space right now. One of them (Voyager 1) is considered to have been in interstellar space since 2012. https://en.wikipe...r_SystemOf course it is wishful thinking to believe that we could improve our technology to do something more than slow unmanned probes. Not in a million years, right?

I read a story a long time ago....Some astronauts were launched on an interstellar trip to a nearby star and it would take a thousand years roundtrip. They would survive using suspended animation and relativity to insure their live return. They risked everything.When they returned 1k years later, mankind had all but colonized the entire solar system and the star they went to was now only 4 days travel time.They had been forgotten about, but when their ship entered the solar system, they were "re-discovered" and treated as heroes by all.Not a bad story.

I would love to see a mission to send a telescope toward Alpha Centauri. Load it onto a rocket and send it off and take pictures along the way to view at different distances of the same Star and system. Or others of interest. We should know more about our neighbors. After all they will be our first endeavors.

"The alternative to this is that planets form in an environment deprived of water—but this would require us to completely rethink our current theories of how planets are born," explained co-author Jonathan Fortney of the University of California, Santa Cruz, USA. "Our results have ruled out the dry scenario, and strongly suggest that it's simply clouds hiding the water from prying eyes."

I must confess that I've not seen any current theory of how planets are born, so I'm somewhat puzzled by this statement that they somehow require water for their initial existence.The one theory that does state that all stars and planets arose from water transformation is given by Russel Humphreys. I have not come across any secular theory other than the accretion one and that one doesn't mention the requirement for water at all. Perhaps someone can enlighten me on that issue.

I would love to see a mission to send a telescope toward Alpha Centauri. Load it onto a rocket and send it off and take pictures along the way to view at different distances of the same Star and system. Or others of interest. We should know more about our neighbors. After all they will be our first endeavors.

Those missions are underway, and have been for decades.....the first probes launched through our solar system are now moving into interstellar space. Of course they don't have the sophisticated instrumentation we could send today, but then nothing we can send today will catch up with them either. We have sent other behind them as well - saw some nice pictures of Pluto the other day....

"The alternative to this is that planets form in an environment deprived of water—but this would require us to completely rethink our current theories of how planets are born,"

I must confess that I've not seen any current theory of how planets are born, so I'm somewhat puzzled by this statement that they somehow require water for their initial existence.

They don't and the text does not state this.

The one theory that does state that all stars and planets arose from water transformation is given by Russel Humphreys. I have not come across any secular theory other than the accretion one and that one doesn't mention the requirement for water at all. Perhaps someone can enlighten me on that issue.

You don't know anything on planet formation but you do know the obscure Russel Humphreys.How come ?

When they returned 1k years later, mankind had all but colonized the entire solar system and the star they went to was now only 4 days travel time.

There's actually serious scientific papers out there about when it makes sense to start building ships to travel to other stars based on estimates of better technology vs. travel time saved.

I.e. if you launch now at speed x, but can expect that spaceships with speed 2x will be developed before you have reached the halfway point then it's better to wait. Of course with ever increasing potential speeds the saved time intervals will shrink. At some point you will get a break-even between expected time saved vs. time for developing a faster drive. That's the point at which you should launch.

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